Safe load indicator for crane hoists



-Sept.21,1943.. I H. P. KUEHNI. I 2,330,060

SAFE LOAD INDICATOR FOR CRANE HOISTS Filed 001:. 28, 1941 Inventor: Hans F? Kuehni,

His Attorney Patented Sept. 21, 1943 ES PATENT OFFICE ssrn LOAD nvmcaroa roa cam nors'rs Hans r. Kuelmi, Schenectady, N. Y., asslgnor to General Electric Company, a corporation of New York Application October 2a, 1941, Serial No. 416.841

8 Claims.

My invention relates to a sale load indicator for crane hoists, the object being to provide a crane operator withmeans which will enable him to avoid so' loading his crane as to tip the crane from its tracker support due to the leverage action oi. the loaded boom taking into conierred electric measuring circuit for carrying out.

theinvention, and Fig. 3 a simplified electric circuit for this purpose.

In Fig. l I have represented a crane hoist which will serve to represent the general type oi crane to which my invention may be applied.

In such illustration, l may represent a stationary or movable support having a circular track ii thereon. A crane I2 i rotatively mounted on the circular track byfianged wheels II. The crane is shown provided with a boom i4 hinged at II. A cable ll connected between a motor operated drum I1 and the boom I4 is provided for raising-and lowering the boom. Another caber i8 has one end secured in fixed relation to the outer end of the boom, runs through the hoisting hook pulley ll back over a pulley Ill at the outer end of the boom and to a motor operated hoist. drum 2|. The rotation of the crane about track ii and the raising and lowering oi the boom i4 and the hoisting hook 22 is under the control of the operator indicated at 23. The crane is assumed to be electrically operated but this is immaterial to my invention. It is possible for an operator to so load a crane of this general type as to tip the crane from its track and cause damage, loss of time, etc.

In general, we may say that the stability of the crane is determined by the moment of crane load at P times horizontal boom radius D. Thus, ii the boom makes an angle X to the vertical, agiven load P will produce a tip moment:

culation or measurement. It is desirable for the operator to have before him an instrument or signal or both at which he may look at or be warned by in any case where the loading of the crane approached an unsafe or tipping condition. In accordancew'ith my invention I provide the operator with an instrument 24 and an alarm 25 conveniently located which indicates the measurement of the tip moment M and gives an alarm in case M approaches dangerously close to the value K.

For the measurement of the load P, I provide an electro-responsive gauging device 25 in electric device at 2'! having a part rotated with and responsive to the turning of-the boom it about its pivot l5.

Electric currents proportional to these measurements P and sin X are conveyed to the electrical instrument at 24, in the form of a wattmeter, where the products of the currents are measured in terms of M. The meter isprovided with alarm contacts which close to actuate the alarm." in case the measurement approaches dangerously close to the value K.

A preferred measuring system for this purpose is shown in Fig. 2. The load measuring device 28 comprises a known form of electric strain gauge. It comprises a piece of metal 28 of suitable dimensions and material which is placed under tension by the load or pull on cable 18 and which is elongated within its elastic limit in Proportion to such tension. Fastened near the opposite ends of the tension member 2-8 are the p two magnetic core parts 2-9 and 30 of a variable For any particular crane there is an optimum moment which, it exceeded. will tip the crane over. This optimum moment we may designate Kl Hence, in any situation P(D sin x) must not exceed K. K may be readily determined by calreactance. These parts are, therefore, moved towards and away from each other with decrease and increase of the load P. such variation changes the air gap in the magnetic circuit and the current in the coil II which is energized by alternating current and produces a flux across the air' gap between the magnetic core parts.

meter 24. Elongation 01 member 28 due to load P unbalances the bridge and a current is supplied to the wattmeter proportional to P.

The device 21 for measuring sin X may comprise a rotary type alternating current voltage regulating transformer having a stationary primary winding 31 and a rotary secondary winding 38. The rotary winding 28 will be rotated by the rotation of boom l4 about its axis IS. The winding has opposite points energized from the A.-C. source 35 and the secondary winding ll has diametrically opposite terminals and feeds through a rectifier 39 to the remaining coilof the D.-C. wattmeter 24.

If the boom [4 were straight up and the angle X zero. rotor 38 would be so positioned with respect to stator 3! that the leads to the respective 4 windings would be 90 degrees displaced from 5 each other. Under these conditions no voltage would be induced into the secondary winding 38. With the boom i4 lowered to the horizontal, the rotor 38 would be rotated to have its leads in line with the primary connections and a maximum voltage wouldbe induced into the secondary. Thus the device 21 produces a voltage proportional, to sin X, as intended.

For calibration purposes and to meet the conditions of different cranes the circuits areprovlded with calibration adjustment features. For example, the reactance 34 may be adjusted so that a small D.-C. voltage appears across the rectifier 36 when there is no load on the hook 22 to represent the load of the weight of the boom assembly itself. One of the leads to the stator winding 31 may be adjustable to introduce a small plus or minus voltage on the rectifier 39 to adjust for cranes where the pivot of the boom is not directly over the tipping point which in, 5 this case is theright-hand wheel l3. A variable. resistance 40 may be provided in the leads to the rectifier 39 to obtain the desired relation between the currents supplied from the two rectifiers to the wattmeter. I

Exacting calibration of thewattmeter is not important except for values 01' M close to the critical tipping moment K. Calibration may be had by trial for example with the boom horizontal and with P hooked to an excessive load on the ground, tighten cable I8 until the lefthand wheel i3 is raised slightly from the track.

The reading of the meter 24 then corresponds to K and the alarm contacts at 4| should be set to ring the alarm and the scale of the meter at this pointer position and above may be painted, red for example as indicated at 42.

Another trial should be made with the boom l4 nearly vertical. Trials may also be made in lifting loads with maximum acceleration, etc. During such calibration trials a safety cable such as represented at 43 may be used to prevent dan-' gerous tipping. The meter 24 is marked and its 2,sao,oco

enough to the danger point so that the operator will utilize the full safe capacity of his crane.

A less elaborate measuring circuit for ro gh work is represented in Pig. 3. A reactance bridge similar tothe one of Fig. 2 is provided but is energized through the deviceil, and an A.-C. voltmeter 43-is connected across the bridge. The voltmeter serves the purpose of wattmeter 24 of Fig. 2. The components sin X and P are combined in the bridge instead of in. the wattmeter wattmeter for measuring the product of suchcurrent and producing an indication in terms of the degree of safety of crane boom loading. I

2. Apparatus for indicating the safe loading of boom cranes comprising an electrical measuring instrument, a pair of electric controllers jointly influencing the energization of said instrument. one of said controllers causing the energization of said instrument to be substantially. proportional to the boom loading and the other of said controllers causing the energization of said instrument to be substantially proportional to the sine of the angle of the boom from a vertical position, said instrument having a measurement response proportional to the product of the boom loading and the sine of the angle of the boom from a vertical position.

3'. Apparatus for indicating the saf loading 40 of boom cranes comprising an electrical measuring instrument, a strain gauge associated with an energizing circuit of said instrument, said gauge being controlled by the loading of the crane boom and causing the energizationof said instrument to be substantially proportional to the boom loading, and a voltage regulator asso-' ciated with an energizing circuit of said instrument. said regulator having relatively rotatable primar and secondary windings, one or which isrotated by the raising and lowering of thehoom to cause the energization of said instrument to be substantially proportional to the angle of said boom from a vertical-position, said instru-- position.

HANS P. KUEHNI.

as new and desire to secure by r 

